1. Field of the Invention
This invention relates to a display panel and a display apparatus. More particularly, this invention relates to a display panel comprising a plurality of pixels formed by using organic electroluminescent devices (organic EL devices) and also to a display system comprising such a display panel.
2. Related Background Art
Organic EL devices are characterized by field luminescence of a thin film multilayer structure capable of emitting light to an enhanced degree of luminance. Organic EL devices can emit light highly efficiently if a low voltage is applied thereto particularly when the number of functional organic layers including at least a luminescent layer of an organic luminescent material is increased (“Applied Physics Letters,” Vol. 51, 1987, p. 913; Vol. 65, 1989, p. 3610). Each device has a basic structure of anode/hole transport layer/luminescent layer (containing an organic luminescent material)/cathode. In the course of time, an alternative basic structure of anode/hole transport layer/luminescent layer/electron transport layer/cathode was proposed to further improve the efficiency. Meanwhile, efforts have been made to improve the emission efficiency by arranging a blocking layer between the luminescent layer and the electron transport layer for the purpose of blocking carriers passing through the luminescent layer or by arranging a metal thin film as electron injection layer between the cathode and the electron transport layer so that carriers may be injected with a low voltage.
In recent years, attention has been paid to luminescent devices including those made of an Ir complex to utilize the luminescence of triplet excitons showing a high occupancy ratio of 75% (single excitons being responsible for the remaining 25%) of electrons and holes injected from electrodes (“Applied Physics Letters,” Vol. 75, 1999, p. 5). Fluorescence is used for luminescence caused by transition from an excited state of singlet, whereas phosphorescence is used for luminescence caused by transition from an excited state of triplet.
For example, the chemical compound having the structural formula shown below is an EL material Alq3 developed by C. W. Tang et al. of Kodak Co. and adapted to mainly use fluorescence. Alq3 emits green light and is adapted to produce fluorescence from a singlet state. 
On the other hand, the compound having the structural formula shown below is an electroluminescent material Ir(ppy)3 that was developed by M. A. Baldo et al. of Princeton University and emits green light just as Alq3. It is adapted to produce phosphorescence from a triplet state and can provide efficiency several times greater than that of Alq3. 
Organic EL devices such as those described above are adapted to field luminescence to an enhanced degree of luminance when an electric current flowing between electrodes or from a thin film transistor (to be referred simply as TFT hereinafter) is used. Therefore, they are expected to find applications in the field of high density display apparatus. Full-color thin film displays can be realized by using organic EL devices that are adapted to emit red, green and blue (RGB) lights.
While the devices of display apparatus that utilize fluorescence as described above show a low efficiency, they response quickly when they decrease light. Some of them (typically made of an electroluminescent material such as Alq3) show a light decreasing delay of about tens of several nano-seconds.
On the other hand, while the devices of display apparatus that utilize phosphorescence as described above show a high efficiency, some of them (typically made of an electroluminescent material such as Ir(ppy)3) show a light decreasing delay between about 0.8 micro-seconds and several milli-seconds.
The luminescence efficiency of a phosphorescent material can remarkably differ from that of a fluorescent material in the initial states because of the respective modes of energy transition of excitons.
In the case of electroluminescence attributable to phosphorescence, carriers are firstly excited to a state above the lowest excited state of singlet and the lowest excited state of triplet. Subsequently, an internal conversion from the lowest excited state of singlet to the lowest excited state of triplet takes place and carriers tend to go back to the ground state, emitting luminescence energy. On the other hand, in the case of electroluminescence attributable to fluorescence, while excitation occurs as in the case of electroluminescence attributable to phosphorescence, carriers that are excited to above the lowest excited state of triplet emit energy as heat. In view of the above described mechanisms of electroluminescence, it may be so concluded that electroluminescence attributable to phosphorescence and electroluminescence attributable to fluorescence differ from each other particularly in terms of service life and the difference of service life is caused by the difference of energy transition among molecules and the resultant difference in terms of emission of heat and morphological changes.
Thus, due to the difference of electroluminescent characteristics between fluorescent materials and phosphorescent materials, efforts have been paid to develop high luminance high quality display panels by arranging pixels made of a single or identical material, which may be a fluorescent material or a phosphorescent material, on the display panel. These days, display apparatus formed by using such display panels are required to perform better than ever.